1. Field of the Invention
This invention relates to processing a pair of substantially identical signals contaminated by uncorrelated noise to produce a like signal with an improved signal-to-noise ratio.
2. Description of the Prior Art
When two substantially identical signals contaminated by uncorrelated noise are received over two channels, a like signal with an improved signal-to-noise ratio (SNR) can be realized by linearly combining the contaminated signals. This may be appreciated by considering the following "left" and "right" contaminated signals EQU l = s + n.sub.l and EQU r = s + n.sub.r, respectively,
where s is the signal amplitude (with power S), n.sub.l is a "left" noise amplitude (with power L), and n.sub.r is a "right" noise amplitude (with power R). The SNRs of these two contaminated signals are EQU SNR.sub.l = S/L and EQU SNR.sub.r = S/R, respectively.
The average of the two contaminated signals (l + r)/2 has a signal power S and, if the two noises are uncorrelated, a noise power of (R + L)/4. The SNR of the average channel output is, therefore, EQU SNR.sub.a = 4 (S/L+R).
if the two uncorrelated noises have equal power (L = R), then EQU SNR.sub.a = 2 (S/L),
a situation usually referred to as a "3-dB improvement in SNR." This 3-dB gain in SNR is in fact the maximum achievable through linear processing.
By contrast, a human listener seems to be able to do much better when processing his two-channel ear inputs. If human performance is measured by speech intelligibility, for example, binaural performance may exceed monaural performance (of the "better" ear if they are different) by as much as 12 dB or more. This has often been referred to colloquially as the "cocktail-party" effect which derives its name from the ability to pick out a single talker from a loud babble of spatially dispersed voices -- a situation frequently encountered at cocktail parties. The fact that a human can understand speech under such circumstances, implies binaural signal-processing capabilities which go far beyond the 3-dB improvement obtainable from linear processing. Technically this phenomenon is often referred to as "binaural release from masking" and is measured by the "binaural masking level difference" (BMLD). What nonlinear processes a human uses to accomplish this feat, however, is not known in any detail, although "contra-lateral" neural inhibition and excitation processes in the auditory pathways between the ears and the human auditory cortex are probably involved.